The disclosure relates to a battery cell unit with a battery cell and a monitoring device for monitoring the state of the battery cell. The disclosure also relates to a method for determining a complex impedance of a battery cell arranged in a battery cell unit. The disclosure further relates to a vehicle with a battery system which has a battery having a plurality of battery cell units according to the disclosure.
It is conventional to refer to batteries for use in hybrid and electric vehicles as traction batteries since said batteries are used to supply electrical drives. In order to achieve the power and energy data required in the case of hybrid and electric vehicles, individual battery cells in the traction batteries used are connected in series and, in some case, also in parallel. In the case of electric vehicles, often 100 battery cells or more are interconnected in series, with the result that battery voltages of up to 450 V can arise. Also, in the case of hybrid vehicles, the voltage limit of 60 V, which is still rated as noncritical in the case of touching contact by a person, is usually significantly exceeded.
FIG. 1 illustrates the basic circuit diagram of a battery system 10 with such a traction battery 20. The battery 20 comprises a plurality of battery cells 21. In order to simplify the illustration in FIG. 1, only two battery cells are provided with the reference sign 21.
The battery 20 is formed from two battery cell series circuits 22, 23, which comprise in each case a plurality of series-connected battery cells 21. The battery cell series circuits 22, 23 are connected by their connections in each case to a battery terminal 24, 25 and to a connection of a service plug 30. The positive battery terminal 24 is connectable to the battery 20 via a disconnecting and charging unit 40, which comprises a switch disconnector 41 which is connected in parallel with a series circuit composed of a charging switch 42 and a charging resistor 43. The negative battery terminal 25 is connectable to the battery 20 via a disconnecting unit 50 which comprises a further switch disconnector 51.
FIG. 2 shows a diagram 60, which illustrates in a highly schematic manner the fault mechanisms 61 of lithium-ion batteries and their consequences 62. The illustrated fault mechanisms 61 can lead to thermal runaway 64 of the battery cells 21 caused by an impermissible increase 63 in temperature. In the event of the presence of a thermal runaway 64, an emission of gas 65, which, for example, can arise on opening of a rupture valve as a consequence of increased battery cell internal pressure, a fire 66 in the battery cells or, in an extreme case, even rupture 67 of the battery cells 21 can occur. Therefore, the occurrence of thermal runaway 64 when using the battery cells 21 in traction batteries needs to be ruled out with the greatest possible probability of close to 1.
Thermal runaway 64 can occur in the case of overcharging 70 of a battery cell, as a consequence of deep discharge 80 of a battery cell 21 during the subsequent charging operation or in the event of the presence of impermissibly high charging and discharge currents of the battery cell 21 which can result from an external short circuit 90, for example. In addition, thermal runaway 64 can also occur in the event of the presence of a battery cell-internal short circuit 100, which can arise, for example, as a consequence of a severe mechanical force effect during an accident 101 or as a consequence of the formation of battery cell-internal dendrites 102, which can arise, for example, in the event of the presence of excessively high charging currents at low temperatures. Furthermore, thermal runaway 64 can also occur as a result of battery cell-internal short circuits which can be caused by impurities in the battery cells 21 resulting during manufacture, in particular by metallic foreign particles 103 present in the battery cells 21. Thermal runaway 64 can also occur in the event of the presence of impermissible heating of the battery cells 21 which can arise, for example, as a consequence of a vehicle fire or in the event of the presence of an overload 120 of the battery cells 21.
Safety tests are prescribed for lithium-ion battery cells. In order to be able to transport the battery cells 21, for example, UN transport tests must be performed. The test results must be assessed according to the EUCAR Hazard Levels. Here, the battery cells 21 must meet predefined minimum safety levels. In order to achieve this, comprehensive additional measures are found in battery cells 21 which are designed for use in traction batteries. Such additional measures are met such that so-called safety devices are integrated in the battery cells. The safety devices specified in the following text are typically integrated.
An overcharge safety device (OSD) is integrated in a battery cell 21. Such an overcharge safety device has the effect that the battery cell 21 does not exceed an EUCAR hazard level 4 during an overcharging operation. The permissible range for the battery cell voltage ends at 4.2 V. In the case of an overcharging operation, above a battery cell voltage of approximately 5 V, such a high internal pressure builds up in the battery cell 21 that a membrane of the overcharge safety device curves outwards and the battery cell 21 is electrically short-circuited. As a result of this, the battery cell 21 is discharged until a battery cell-internal fuse is activated. The short circuit in the battery cell 21 between the two battery cell terminals is maintained via the overcharge safety device.
A battery cell fuse is also integrated in a battery cell 21. This fuse integrated in the battery cell 21 is a very effective protective instrument on a battery cell level, but causes considerable problems when using the battery cells 21 to construct a series circuit in a battery module or in a battery system. In these cases these measures are rather counterproductive.
A nail penetration safety device (NDS) is often also integrated in a battery cell 21. Said nail penetration safety device protects the battery cell 21 by virtue of a defined short-circuit path which does not result in such severe local heating of the battery cell in the region of the nail penetration that local melting of the separator provided could result being constructed when a nail penetrates into the battery cell 21.
A safety function layer (SFL) is also integrated in a battery cell 21. The safety function layer is realized by the ceramic coating of one of the two electrodes, preferably by the ceramic coating of the anode. In the event of melting of the separator, an areal short circuit of the battery cell 21 and therefore extremely rapid conversion of the electrical energy from the battery cell 21 into lost heat can be prevented by means of the safety function layer.
A crush safety device is in addition also integrated in a battery cell 21. The crush safety device has a similar mode of operation to the nail penetration safety device. In the event of a severe mechanical deformation of the battery cell housing, a defined short-circuit path is provided in the battery cell 21 which prevents severe local heating of the battery cell 21 and thus increases the safety of the battery cell 21.
In the battery cells 21 under development at present, in particular the measures for the electrical safety which protect against overcharging, for example, or ensure overcurrent protection are associated with considerable complexity. In addition, these measures tend to be even rather counterproductive instead of expedient once a battery cell 21 is used in a battery module or in a battery system. For example, on activation of the fuse of a battery cell 21, the situation may arise whereby the electronics of the existing battery management system (BMS) are subject to very high negative voltages. This results in additional complexity on the battery system level since the transport regulations at the battery cell level need to be adhered to without any benefit being associated with this.
FIG. 3 illustrates the basic circuit diagram of a battery system 10 known from the prior art which comprises a traction battery 20 with a plurality of battery cells 21 and a battery management system. The electronics of the battery management system (BMS) have a decentralized architecture, in which the cell monitoring units 130 formed from the monitoring electronics (CSC electronics) of the battery cells 21 are in the form of satellites, are each provided for monitoring the function state of one or more battery cells 21 and communicate with a central battery control device (BCU) 140 via an internal bus system 141.
The electronics of the battery management system, in particular the monitoring electronics of the battery cells 21, are necessary in order to protect the battery cells 21 from the critical states illustrated in FIG. 2, which can result in thermal runaway. A high degree of complexity is involved in the electronics of the battery management system in order firstly to protect the battery cells 21 from overload due to external causes such as, for example, due to a short circuit in the inverter of an electric drive, and secondly to avoid a situation whereby the battery cells are endangered by malfunction of the electronics of the battery management system, such as, for example, by faulty detection of the battery cell voltages by the cell monitoring units 130.
As is the case for the battery system 10 illustrated in FIG. 1, in the battery system 10 illustrated in FIG. 3 the traction battery 20 is connectable to a positive battery terminal 24 via a disconnecting and charging device 40 and is connectable to a negative battery terminal 25 via a disconnecting device 50. In this case, in each case the same reference signs have been used for denoting identical components for the battery systems illustrated in FIGS. 1 and 3.
In addition, the central battery control device 140 is designed to actuate the switch disconnector (relay) 41 and the charging switch (relay) 42 of the disconnecting and charging device 40. The actuation of the switch disconnector 41 and the charging switch 42 by means of the battery control device 140 is symbolized by the arrow 142 here. The central battery control device 140 is also designed to actuate the further switch disconnector (relay) 51 of the disconnecting device 50. The actuation of the switch disconnector 51 by means of the battery control device 140 is symbolized by the arrow 143.
The central battery control device 140 is connected to a respective other battery terminal 24, 25 in each case via a high-voltage line 144, 145. In addition, the central battery control device 140 comprises current sensors 150, 160, which are provided for measuring the current flowing through the traction battery 20. The battery control device 140 also communicates with a vehicle interface via a CAN bus 146. Information relating to the function state of the vehicle can be provided to the battery control device 140 via the CAN bus.
When using a battery management system of a battery system known from the prior art, it is therefore desired to increase the safety of the battery system 10 such that no unreasonable risk occurs. In doing so, pursuant to ISO 26262, stringent requirements are placed on the functional safety of the battery management system since a malfunction of the electronics can result in a risk. For battery management systems in electric vehicles and plug-in hybrids, presumably grading in accordance with the hazard level ASIL C will be established if the safety of the battery cells 21 cannot be significantly increased.